(1) Field of the Invention
This invention relates to a hot-melt polyamide adhesive composition comprising a polyamide as a base, and a composite metal material formed by using this adhesive composition.
(2) Description of the Related Art
Although polyamide resins are effective for bonding surfaces of metal materials such as steel plates and aluminum plates and various other materials such as glass, ceramic, and plastic materials, research is being carried into ways of improving the properties of these polyamides.
When a homopolyamide is used alone as the polyamide resin, the adhesion strength is poor, and accordingly, various proposals have been made to improve the adhesive characteristics of this resin. For example, the following methods have been proposed: a modification method wherein a reactive monomer is added at the step of preparing a polyamide resin to obtain a modified polyamide resin such as a copolymer or terpolymer; a method wherein a primer such as an epoxy type primer is applied prior to the application of a polyamide resin adhesive; and a method wherein a third component is used together with a polyamide resin.
When a modified polyamide resin such as a copolymer or terpolymer is used, the excellent heat resistance which is one of the characteristics of an unmodified polyamide resin, tends to decrease, and the melting point of the modified polyamide resin is lower than that of the unmodified polyamide resin. Accordingly, when a bonded material such as a bonded steel plate is subjected to a high-temperature treatment such as a bake-coating treatment, peeling sometimes occurs in the bonded portion. In the method in which a primer is costed on the surface of an adherend and bonding is then effected with an adhesive, problems such as a reduction of the operation efficiency, prolongation of the bonding time, reduction of the peel strength at a high temperature, reduction of the flexibility, and increase of the cost often arise, and accordingly, this method is not preferred from the practical viewpoint.
As another improved hot-melt polyamide resin adhesive, there can be mentioned an adhesive composition formed by incorporating polyethylene into a polyamide resin (Japanese Unexamined Patent Publication No. 51-59,936), a can-forming adhesive composition formed by incorporating an ionomer into a polyamide resin (Japanese Unexamined Patent Publication No. 51-67833), and an adhesive composition formed by incorporating a specific organic silicon compound into a polyamide resin (Japanese Unexamined Patent Publication No. 48-92,439).
The properties of a polyamide resin as a hot-melt adhesive can be improved to some extent by the addition of an additive as mentioned above. In many cases, however, the surface of an adherend must be pre-treated with a primer, and even when the foregoing adhesive compositions are used, it is almost impossible to obtain a sufficiently strong bonding without that primer treatment.